inp = raw_input('Enter score: ')
try:
 score = float(inp)

 if score > 1.0 or score < 0.0:
   print 'Score out of range'
 elif score >= 0.9:
   print 'A'
 elif score >= 0.8:
   print 'B'
 elif score >= 0.7:
   print 'C'
 elif score >= 0.6:
   print 'D'
 else:
   print 'F'

except:
    print "Please enter a number between 0 and 1"